In non-nursing postpartum women, ovulation resumes between 2 and 3 months after delivery, but is delayed if breastfeeding occurs. Often fertile menstrual cycles recur in nursing western women before the end of the first year. In contrast, in developing countries, interbirth intervals of nearly four years have been attributed to the inhibitory effects of the suckling stimulus. In fact, worldwide, breastfeeding remains a significant determinant in the rate of population growth. The differences in fecundity in these two groups of nursing women is attributable to cultural practices which diminish the frequency and duration of the suckling episodes, particularly at night. Old world monkeys similarly demonstrate postpartum infertility. Because of the similarities in reproductive endocrinology of women and old world monkeys, the latter are excellent research models for women. In this proposal rhesus monkeys, of the Caribbean Primate Research Center, will be utilized in investigations of postpartum anovulation. The specific aims of the studies are 1) to determine the role of chorionic gonadotropin, estradiol and progesterone during early pregnancy in inducing the inhibition of the hypothalamic-pituitary-ovarian axis, 2) to determine the effect of nighttime suckling on the maintenance of lactational anovulation and amenorrhea, 3) to determine the effects of nursing behavior on gonadotropin secretion, and 4) to determine the patterns of gonadotropin secretion during the H-P-O refractory interval in non-nursing postpartum monkeys. The experimental models will utilize 1) simulation of pregnancy hormonal concentrations, 2) restriction of mother-infant interaction, particularly at night, and 3) monitoring of LH and FSH secretion during the postpartum interval to accomplish these objectives. These results will have implications for improving breast-feeding practices, so that the maximal contraceptive effect is realized, for developing new contraceptive practices based upon the physiologic anovulation occuring during the postpartum intervals, and for the treatment of infertilities which are caused by endocrinopathies, similar to the normal processes underlying postpartum anovulation.